1. Field of the Invention
The present invention relates to new aqueous coating compositions for the production of metallic base lacquers wherein the binder is based on polyurethane polyureas containing a minimum amount of carbonate groups and a maximum amount of urethane and urea groups, and to the coatings produced from these coating compositions.
2. Description of the Prior Art
The coating of motor vehicles with so-called metallic coatings has in recent years increasingly replaced coating in plain colors. The classical one layer coating has therefore progressively given way to the formation of an intermediate build up of layers of metal pigmented base coats and clear top coats.
These base coats must be highly diluted and, therefore, make a relatively very large contribution to the total solvent emission from a coating line. There has therefore been a trend to use base coats in which the binders are soluble or dispersible in water. Such binder solutions or dispersions must be suitable for use in preparing:
the first (original) coating applied by the manufacturer in which the coating composition is used in stoving systems, and PA0 a repair coating which is applied to the vehicle after its manufacture and is therefore limited to the temperatures obtained during forced drying (i.e., .ltoreq.90.degree. C.). PA0 (i) at least 200 milliequivalents per 100 g of solids of chemically incorporated carbonate groups --O--CO--O-- and PA0 (ii) a combined total of up to 320 milliequivalents per 100 g of solids of chemically incorporated urethane groups --NH--CO--O-- and chemically incorporated urea groups --NH--CO--NH--. PA0 a) organic polyisocyanates which contain no hydrophilic groups or groups convertible into hydrophilic groups with PA0 b) relatively high molecular weight organic polyhydroxyl compounds which have no hydrophilic groups or groups convertible into hydrophilic groups, PA0 c) optionally low molecular weight compositions containing at least two isocyanate reactive groups but no hydrophilic groups or groups capable of conversion into hydrophilic groups, PA0 d) optionally non-ionic hydrophilic starting components containing at least one isocyanate group or at least one isocyanate reactive group and PA0 e) Optionally starting components containing at least one ionic group or at least one group capable of conversion into an ionic group as well as at least one isocyanate reactive hydrogen atom, PA0 1. dihydroxy polyesters obtained from dicarboxylic acids such as succinic acid, adipic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid and from diols such as ethylene glycol, propane-1,2-diol, propane-1,3-diol, diethylene glycol, butane-1,4-diol, hexane-1,6-diol, octane-1,8-diol, neopentyl glycol, 2-methyl propane-1,3-diol and the various isomeric bis-hydroxymethyl cyclohexanes; PA0 2. polylactones such as the polymers of .epsilon.-caprolactone initiated with the above mentioned dihydric alcohols; and PA0 3. polyethers, e.g., the polymers or copolymers of tetrahydrofuran, styrene oxide, propylene oxide, ethylene oxide, the butylene oxides or epichlorohydrin initiated with divalent starter molecules such as water, the above mentioned diols or amines containing 2 NH bonds, in particular the polymers and copolymers of propylene oxide and optionally ethylene oxide. Ethylene oxide may be used as a portion of the total quantity of ether molecules, provided the resulting polyether diol contains not more than 10% by weight of ethylene oxide units. It is preferred to use polyether diols which have been obtained without the addition of ethylene oxide, especially those based on propylene oxide and tetrahydrofuran alone. PA0 d1) diisocyanates and/or compositions which contain isocyanate reactive hydrogen atoms and are difunctional in isocyanate polyaddition reactions, the diisocyanates and compositions also containing polyether side chains containing ethylene oxide units, and PA0 d2) monoisocyanates and/or compositions which are monofunctional in isocyanate polyaddition reactions and contain an isocyanate reactive hydrogen atom, the monoisocyanates and compositions also containing terminal polyether chains containing ethylene oxide units and PA0 d3) mixtures of dI) and d2). PA0 R' represents hydrogen or a monovalent hydrocarbon group having 1 to 8 carbon atoms, preferably hydrogen or a methyl group, PA0 R" represents a monovalent hydrocarbon group having 1 to 12 carbon atoms, preferably an unsubstituted alkyl group having 1 to 4 carbon atoms, PA0 X represents a group obtained by removing the terminal oxygen atom from a polyalkylene oxide chain having 5 to 90, preferably 20 to 70 chain members, which chain members contain at least 40%, preferably at least 65%, of ethylene oxide units and may in addition contain propylene oxide, butylene oxide or styrene oxide units, preferably propylene oxide units, PA0 Y represents oxygen or NR"' wherein R"' has the same definition as R" and PA0 Z represents a group which corresponds to the definition of Y.
The use of aqueous binders as metallic base coats is known. For example, DE-OS 3,210,051 describes metallic base coats in which the binders are based on water dispersible polyurethane polyureas which can be crosslinked with melamine formaldehyde resins or blocked polyisocyanates with the aid of heat after the base coat has been applied. EP-A-256,540 discloses aqueous mixtures of polymers containing hydroxyl groups and hydrophilic polyurethane polyureas which are substantially identical to those just described and may also be cross-linked by this method.
According to EP-A-260,447, polycondensates are used as mixtures with conventional polyurethane dispersions. The polycondensates contain carboxyl and epoxy groups, are self cross-linking and therefore do not require the addition of melamine resins.
Similar to EP-A-256,540, binders for metallic multilayered coatings based on mixtures of hydroxyl-containing polymers and hydrophilic polyurethane polyureas are described in EP-A-297,576; however, the polymers for these binders are prepared by emulsion polymerization of their monomers in the aqueous polyurethane polyurea dispersion used as carrier medium.
Polyurethane dispersions in which the dispersed phase is based on cross-linked polyurethanes are described in DE-OS 3,545,618 and in DE-OS 3,606,513. In the former, cross-linking is occurs before dispersion in water, while in the latter the polyurethane is crosslinked by heating after conversion into an aqueous dispersion.
Common to all of these known polyurethanes and polyurethane polyureas is the fact that the macrodiols used for their preparation are polyether polyols or preferably polyester diols and, in particular, polylactone diols. The drying and cross-linking temperatures mentioned in the prior publications generally range from about 80.degree. C. to 140.degree. C., which satisfies the condition mentioned above that the binders must be suitable for two different uses, i.e., as stoving lacquer binders used for the original coating in the manufacture of the car body (stoving temperatures above 140.degree. C.) and as binders for repair lacquers which may be used, for example, for recoating individual faulty patches on the already assembled vehicle and which are cured at about 80.degree. C. It must be possible to use the same lacquer formulation for the repair coating as that used for the stoving lacquer, both for economic reasons and to ensure absolute identity of color and surface effect. It is also essential that the most important coatings, e.g., adhesion and weather resistance, must also be obtained in the repair coatings.
One particularly important property is the resistance of the coating to condensation water. This property is determined according to DIN 50,017 by exposing a coated test sample to condensing water vapor at about 40.degree. C. in an air conditioning chamber. After a predetermined length of time, the test sample is examined for the formation of blisters which are an indication of the loss of adhesion between the coating and the substrate due to moisture penetrating the lacquer film. The brilliance (or "distinctness of image" (DOI)) is also determined by this test and should not undergo any significant deterioration. Lacquer films which have been cured at low temperatures in the drying process have particular difficulty passing this test, especially if they have been formed from aqueous dispersions. Thus polyurethanes from the aqueous phase which are commercially available generally only moderately pass this test and in some cases fail completely.
Another essential requirement for the repair coating is when the metallic base coat is cured at low temperature, it must adhere firmly to the previously stoved top coat lacquer. The reason for this is that if any areas of coated surface on a finished part are damaged during assembly, the faulty patches are generally rubbed down only over a small area with a suitable abrasive. This either completely removes the top coat of lacquer or roughens it up sufficiently to enable the fresh layer of lacquer to adhere firmly. However, in order to ensure optical continuity the repairing base coat is sprayed over a wide area on and adjacent to the faulty patch and therefore over a considerable area of top coat lacquer which has not been rubbed down. The repair coating must adhere just as firmly to this area as to the original surface layer. The adhesion is determined in practice by the grid section method (according to s DIN 151). The known polyurethane coatings do not satisfy the above requirement satisfactorily, if at all.
It is an object of the present invention to provide new aqueous coating compositions for the production of metallic base coats which satisfy the previously mentioned property requirements, i.e., (1) coatings of the same quality are obtained regardless of the curing temperature so that the coating compositions are suitable for both stoving lacquers for the unfinished shell of motor vehicles and as repair lacquers and (2) coatings obtained from the compositions possess good resistance to condensation water and good adhesion regardless of their method of application.
It was surprisingly found that this object could be achieved with the coating compositions according to the present invention. The polyurethane polyureas present as binders or binder components in the coating compositions according to the invention have a minimum carbonate group content and a maximum urethane and urea group content which surprisingly results in the in the desired property improvements.